International Journal of Modern Physics B

Magnetic properties of the two-dimensional t-J model are reviewed. Some of these properties are close to those observed in cuprate perovskites. In particular, the magnetic response of the model is characterized by the intensive peak at the antiferromagnetic wave vector and at some transfer frequency. The peak splits into several incommensurate maxima for lower and higher frequencies. We discuss mechanisms which are responsible for these peculiarities in the model, which might also refer to cuprates.

The role of orbital ordering on metal–insulator transition in transition-metal oxides is investigated by the cluster self-consistent field approach in the strong correlation regime. A clear dependence of the insulating gap of single-particle excitation spectra on the orbital order parameter is found. The thermal fluctuation drives the orbital order–disorder transition, diminishes the gap and leads to the metal–insulator transition. The unusual temperature dependence of the orbital polarization in the orbital insulator is also manifested in the resonant X-ray scattering intensity.

The temperature dependence of the electrical resistivity and magneto-resistance in silver substituted La_0.7Sr_1-xAg_xMnO₃ pellets prepared by a simple pyrophoric method has been investigated between 10 and 450 K for H=0 and 0.8T. Ag substitution enhances the conductivity of this system. Curie temperature (T_C) also increases sharply from ~303 K (x=0.05) to ~363 K (x=0.25). Above the metal-insulator transition temperature (T_MI), the electrical resistivity is well represented by usual adiabatic polaronic transport, while in the ferromagnetic region at low temperatures, the resistivity data shows an excellent agreement to an expression containing the residual resistivity, two-magnon scattering term and the term associated with small-polaron metallic conduction, which involves a relaxation time due to a soft optical phonon mode. Based on a scenario that the doped manganites consist of phase separated ferromagnetic metallic and paramagnetic insulating regions, all the features of the temperature variation of the resistivity between ~50 K and 300 K could be described very well. All the Ag doped samples reveal resistivity minimum, which is strongly field dependent (H=0.8T). These minimum results have been analyzed and the observed field dependent minimum is found to be in excellent agreement to those calculated from the model based on charge carrier tunneling between antiferro-magnetically coupled grains. Field dependence of magnetoresistance at various temperatures below T_c is accounted fairly well by a phenomenological model based on spin polarized tunneling at the grain boundaries. The contributions from the intrinsic part arising from DE mechanism, as well as the part originating from intergranular spin polarized tunneling are also estimated.

Nanoparticles of MnFe₂O₄ have been synthesized under an induced magnetic field by a hydrothermal process. A strong correlation between the cation distribution and the magnetic properties of MnFe₂O₄ nanoparticles has been identified. From the results of Mössbauer spectroscopy it has been found that the nanoparticles synthesized under zero magnetic fields have 21% inversion occupancy in the tetrahedral sites by Fe³⁺ cations, while this value decreases to 10% in the sample formed under a 0.25 T magnetic field. The results explain well the decrease in saturation magnetization from 62.2 emu/g to 52.8 emu/g for the sample formed under a magnetic field. This provides an understanding on how to control the inner structure in order to control the magnetic properties of these spinel ferrite nanoparticles.

Glass stability and the structural role of iron oxide in xFe₂O₃(100-x)[B₂O₃–MO] systems (M=Ca, Sr or Ba, 0≤x≤30 mol%) were estimated from differential thermal analysis. The results suggest structural changes characteristic for the transition from short range to intermediate range order as the cationic field strength of the alkaline earths increases. In contrast, the iron addition has an opposite effect and determines a higher glass stability of these systems.

We develop the invariant eigen-operator (IEO) theory proposed in the preceding study (Phys. Lett. A321 (2004) 75) to the pseudo-invariant eigen-operator (PIEO) method, with which we derive energy-level gap of two generalized Jaynes–Cummings models. The PIEO usually exists in some Hilbert space which is spanned by the eigenvectors of conservative quantities of the dynamic system.

Creep experiments were conducted on Cu–8.5 at.% Al alloy in the intermediate temperature range, 673–873 K, corresponding to 0.46–0.72 T_m, where T_m is the absolute melting temperature. The present analysis reveals the presence of two distinct deformation regions (climb and viscous glide) in the plot of log versus log σ. The implications of these results on the transition from power-law to exponential creep regime are examined. The results indicated that the rate-controlling mechanism for creep is the obstacle-controlled dislocation glide. A phenomenological model is proposed, which assumes that cell boundaries with sub-grains act as sources and obstacles to gliding dislocations.

Consider the Heisenberg model with two-, three-, four-, five-, and six-spin exchanges interactions applied on a hexagon in strong magnetic field for ³He system. The recursive lattice is constructed by adding next shells (branches) of hexagons on vertexes of previous one. The exact rational recursion relations for partition function are obtained on this recursive lattice with Ising model approximation for multiple-spins interaction. This approach gives us an opportunity to describe solid and fluid ³He films absorbed on the surface of graphite which have a structure of two-dimensional regular triangular lattices. Using dynamical system method of one-dimensional rational mapping allows us to research the magnetic behavior of solid and fluid ³He films, depending on the values of the exchanging parameters and magnetic field. The magnetization plateaus m=0 and m/m_sat=2/3 are obtained in low temperatures and different exchange parameters. The bifurcation points corresponding to the second order phase transition are observed. There is one period doubling that occurs in anti-ferromagnetic case at low temperatures. The existence of sublattices in the considered model allows us to get modulated phases.